1. Field of the Invention
The present invention relates to a mechanical persistent current switch and a method for the same which are preferable for a persistent current switch (PCS) for storing electric current in a superconducting coil such as a superconducting magnetic energy storage (SMES) device or the like.
For a magnetically levitated train, a superconducting electromagnetically propelled boat, a superconducting device for medical diagnosis such as magnetic resonance imaging (MRI) device, and a SMES device, a PCS is used in order to run the superconducting magnet in a persistent current mode. Stable conductivity of large amounts of currents, quick switching operation, and a small energy loss are required for a PCS.
2. Description of Related Art
As standard methods to realize a persistent current mode, three different switching methods have been proposed. Those are thermal, magnetic, and mechanical methods. Presently, the thermal method is mainly employed for a persistent current switch of a superconducting coil for a magnetically levitated train and a superconducting electromagnetic propelled boat or the like.
However, a thermal PCS has a problem in that the operating time for switching is long especially when a large amount of transport currents are passed. For example, the off-switching time for a thermal persistent current switch of 10 kA class using a superconducting coil was 180 seconds. The on-switching time of the same switch was even longer and reached 813 seconds. This is simply due to the fact that it takes a long time to drive a PCS in a large volume from the superconducting to the normal conducting state. It takes a longer time to cool the superconducting coil switch from the normal to the superconducting state.
In the case of a magnetically levitated train and a superconducting electromagnetically propelled boat in which the frequency of switching is extremely low, a slow switching time is not a serious problem. The same idea applies to the case of a superconducting device for medical diagnosis. For these applications, a slow switching time and a small energy loss is not a problem, and therefore, research and development of a thermal PCS are actively performed.
However, the number of switching times is large in a SMES device, and hence a thermal PCS is not suitable. A thermal PCS has another problem that a certain amount of currents are flowing through the switch even in a off state, because the switch is in a normal conducting state and the resistance is finite. Thus an energy loss is unavoidable. A magnetic PCS has the same problem.
In contrast, the electric circuit can be completely shut in the off state of a mechanical persistent current switch (MPCS), since it mechanically opens connector terminals. The switching time is also very short for a MPCS, since a switching operation is simply controlled by closing and opening connector terminals. Thus the MPCS is considered to be more suitable for a superconducting magnet in which on/off switching is frequently repeated.
When superconducting materials were used for connecting terminals of a MPCS, it is possible to supply electric currents to a superconducting magnet from a power supply with no resistance. It is also possible to disconnect the electric current perfectly when the switch is off.
Superconducting materials used for a MPCS, however have been metal alloys such as NbTi and the like. The critical temperature Tc of such metal superconductors was so low that refrigeration by liquid helium (temperature: 4.2K) was indispensable, and hence the cost of facilities was a problem.
Because of such a background, the development of a MPCS, which functions sufficiently at high temperatures such as 77.3 K the boiling point of liquid nitrogen and reduces the facility cost to a great extent, has been desired.
In 1987 RE—Ba—Cu—O oxide superconductors (RE is a rare earth element) that showed higher critical temperature Tc than the boiling point of liquid helium were discovered. With this advent, a trial for producing a MPCS was conducted by using such high temperature superconductor (HTSC) as materials for connectors. At the early stage of research, HTSC materials were prepared by sintering methods. The contact resistance between sintered HTSC materials was high; thus a state of superconductivity was not realized. Therefore, the research was discontinued because the development of a MPCS of desired performance was not clearly foreseen.